The use of size presses as coaters wherein a separate coating head applies a uniform thickness coating film to each of the rolls of the size presses and these rolls substantially simultaneously transfer the coating to opposite sides of a web of paper in a nip found between the size press rolls is becoming relatively common practice. For example, size presses of this type using metering rod type coating applicator heads are shown, for example, in U.S. Pat. Nos. 4,848,268 issued Jul. 18, 1989, to Sollinger and 5,159,893 issued Nov. 3, 1992, to Rantanen.
It is also known to use similar system whenever coating is applied to one roll only of a pair of rolls forming a coating nip.
When operating these systems, it is known to apply a lubricating liquid to the coating rolls before the coating head is moved into coating applying position in order to lubricate the surface of the rolls. The roll surfaces are usually formed by a relatively soft material and when the coating head is moved into coating position, metering rods, which meter the output of coating from the head, are pressed against and indent the roll surface, and are rotated at a different peripheral speed than the surface speed of the roll. If no lubricating film is present, the size press rolls are either slowed which in turn generally results in breaking of the paper web or wear or other damage is inflicted on the soft cover of the nip rolls. Generally, water is simply sprayed onto the surface of the roll(s) in an amount to provide the required lubrication.
The above system seems to work satisfactorily when for relatively heavy basis weight paper passing through the nip, i.e. a basis weight paper of over say, about 40 lb per 1,000 ft.sup.2. However, when lighter weight base sheets, which are generally less strong, are run through the machine, using exactly the same technique, the paper either deforms significantly resulting in tracking problems (i.e. the correct direction of movement of the web cannot be maintained) or alternatively, in the web breaking.
Generally, when stinting this size press equipment, the procedure is to move the coating heads into a preload condition wherein the metering rod is pressed lightly against the surface of the press roll and almost immediately thereafter, to abruptly increase the pressure to full pressure and force the metering rod into the final position. This procedure, in many cases, has been found to be detrimental to the roll cover and to cause paper breaks.